We propose that brief exposure to low energy, frequency specific electric fields can inhibit the loss of tissue in a bone deprived of its normal functional milieu. Importantly, the frequency and energy characteristics of these fields would be similar to those levels produced by normal functional activity, and thereby suggest an intrinsic feedback mechanism for the regulation of bone mass by mechanical loading. Preliminary results demonstrate that specific exogenous electric signals - based on physiologic criteria - can be introduced into the bone tissue and serve to retain the normal quiescent remodeling state even in the absence of mechanical strain. The objective of this proposal is to define an effective regime for utilizing electric fields to prevent the structurally deleterious bone loss characteristic of osteoporosis. In this four year study, a systematic series of in vivo experiments will be performed to isolate three principal parameters of electromagnetically induced electric fields: energy, duty cycle, and frequency, and evaluate their potential to control bone remodeling. The model we will use, the functionally isolated turkey ulna, has been developed and demonstrated by the investigators to be an effective model for osteopenia. The ulna in the wings of turkeys can be retained in situ while deprived of normal mechanical function by removal of its articular extremities. The course of the remodeling response to functional deprivation, together with the modulating effect of frequency specific, low energy electromagnetic field exposure, will be monitored in this model by static and dynamic histomorphometry, microradiography, and physical property measurement. As the contralateral ulna remains surgically undisturbed, it will serve as a baseline control. The efficacy of each signal will be determined by their relative ability to inhibit the bone loss consistent with ulnae subject to disuse alone. Finally, the capacity of these signals to influence normal bone tissue will be evaluated by subjecting intact ulnae, not exposed to surgery, to osteoinductive fields. The immediate clinical relevance of our study is towards the prevention of bone loss in the immobilized or disabled patient, the aging and/or postmenopausal population, or those subject to extended exposure to microgravity. Our preliminary results are encouraging since they demonstrate that structurally deleterious bone loss may be prevented and even reversed by short exposure to extremely low energy electric fields. However, it must also be emphasized that treatment of osteopenia will require chronic prophylaxis, and therefore identification of the minimal daily dose sufficient to retain structurally appropriate bone mass is essential to diminish the possibility of aberrant effects catalyzed by years of exposure.